Selectivity in lichen symbioses

A population genetic study will investigate patterns of symbiont selectivity in lichens, the predominant form of fungal symbioses beside (endo-)mykorrhiza. Three lichen species that differ in their propagation mode will be used as a model. A high level of symbiotic interdependency is assumed in the case of co-dispersal of the symbionts via vegetative propagules being soredia or isidia but a lower genetic diversity of the symbionts, whereas the spores of sexual or mitosporic mycobiont species may re-associate more freely with compatible and available algae. We will study whether photobiont diversity is also correlated with genotypic differentiation of the fungal partner. It will also be analysed whether there is a geographic pattern in the diversity and association patterns of symbionts. This model study will result in new insights of the role of symbiont-selectivity for the evolution and establishment of symbioses.

Lichens are unique amongst the various kinds of fungal symbioses with algae. The lichen habit contributed to a substantial evolutionary radiation of ascomycete fungi, and allows the algal partners to grow well under environmental situations that would be unfavourable to them on their own. In this project investigated the symbiont selectivity of lichens in greater detail. We were interested to know how large is the diversity of algal partners in lichens, and whether there are correlations with ecological and geographic conditions, repectively, or with the reproductive strategy of the fungal partner, which is the name-giving partner and dictates the overall structure of the symbiotic organisms. We found out that lichen species differ substantially with respect to the diversity of their algal partners. In several cases we observed that crust-like lichens with rather broad ecological preferences can accept different related algal species, whereas ecologically more specialized lichens associate only with one single algal species. The latter lichens often also include those which develop highly complex growth types. Lichens, which can use several algal partner species, also indicate a correlation with geographic/climatic conditions. We found out, that lichens which occur both at higher altitudes of the Alps and in the Mediterranean have different algal partners in the respective areas. This difference points on the different ecological preferences of the algal species. On the other hand, we noticed that particularly the algae of higher altitudes have an extremely wide geographic distribution, and may also accur in Antarctica. Generally, our studies show that the diversity of lichen algae is strongly underestimated, and that significantly more species exist, than is known so far. In our investigations of closely related lichens, which use only one algal species as symbiont, we were detected a surprising diversity of algal genotypes within lichen populations. On the other hand, the scarcity or ephemerity of free-living algal partners seems to be a problem in nature. Many lichen fungi have solved this issue by joint propagation of both symbiotic partners together. Also in these species we find a diversity of algal genotypes in lichen populations. Even those lichens with joint propagation of partners seem to be enabled to change the symbiotic algal partner, if required. Our results underline a flexibility of partner choice in lichens, which may have contributed substantially to the diversification and to the species richness of the lichen symbioses.